This invention relates generally to large generator constructions, and specifically to a high voltage bushing utilized to pass an electrical conductor through a wall of a generator frame.
A high voltage bushing is used for passing an electrical conductor through a pressure vessel wall of, for example, a large generator, the conductor carrying electricity out of the generator to voltage and power transformers and then to an electrical grid or the like. It is important that such bushings prevent a cooling gas (e.g., hydrogen) inside the pressurized vessel (stator) from leaking out of the vessel through the bushing stator wall interface. In addition, the conductor must be electrically insulated from the pressurized vessel or stator wall. This is achieved by enclosing the conductor inside a porcelain or other insulating sleeve or shell. An annular, sleeve-like metallic bushing also referred to herein as a “bushing flange”) is telescoped over the exterior surface of the porcelain shell and is utilized to attach the porcelain sleeve to the pressure vessel wall. One such high voltage bushing flange is disclosed in commonly-owned U.S. Pat. No. 5,483,023.
Problems associated with such high voltage bushings include: 1) cracking of the porcelain sleeve due to mechanical stresses imparted by the thermally-mismatched bushing flange; 2) leaking of hydrogen gas from inside the generator stator through the bonding seals between the bushing flange and the porcelain shell; 3) micro crack formation of bonding materials induced by, for example, high density epoxies of virgin porosity, thermal-aging excess tensile stresses, thermal cycling, and/or vibrations experienced during operation.
There remains a need, therefore, for an improved high voltage bushing flange that alleviates excess tensile stresses on the porcelain shell for increased service longevity, and that more effectively blocks potential gas leakage pathways through the bonding seals utilized to provide a buffer between the metal bushing flange and the insulating shell such as, but not limited to, a porcelain shell.